This invention relates to compositions for thermistor, and more particularly to compositions for high temperature thermistors.
The conventional thermistors comprise, for example, cobalt oxide, manganese oxide, nickel oxide and/or iron oxide, and they have been usually employed at a temperature range from room temperature to about 300.degree. C. Recently, a high temperature thermistor has been developed which is available for employment in a higher temperature range i.e. up to about 1000.degree. C. For example, Japanese patent publication No. 41-15855/1966 discloses a high temperature thermistor consisting of SnO.sub.2 --TiO.sub.2 solid solution as a principal component and as additives, at least one oxide selected from the group consisting of Sb.sub.2 O.sub.3 and Ta.sub.2 O.sub.5 in an amount of less than 5 % by mole, which can be used at a high temperature up to 1000.degree. C. It is also disclosed in Rev. of Scientific Inst., 40, pp544-549(1969) that a thermistor consisting of ZrO.sub.2 as a principal component and an additive of Y.sub.2 O.sub.3 is available for employment at a high temperature i.e. up to about 1000.degree. C. Furthermore, U.S. Pat. No. 3,598,764 discloses a thermistor consisting of CeO.sub.2 as a principal component and an additive of ZrO.sub.2 in an amount of 2 to 15 % by weight, which is available for employment at a high temperature i.e. up to 1000.degree. C. However, these conventional high temperature thermistor compositions have some disadvantages such at time-dependent characteristics. Particularly, they show unstability when they are used at a high temperature under applied d.c. voltage.
Recently, a high temperature thermistor which can be used in a working temperature above 300.degree. C has been intensively required. For example, a thermistor is desired which can be used as a sensor of a temperature controller in a thermal reactor where exhaust gas from a motor car engine is reburnt. In this case, this thermistor must operate steadily at a temperature range from 600.degree. C to 1000.degree. C, and in addition the high temperature thermistor is used under an application of d.c. voltage because a d.c. electrical battery used as an electric power source in a motor car.